Hydrochars from waste biomass of açaí and curauá as adsorbents for methylene blue using factorial experimental design

Abstract This study investigated the performance of four hydrochars derived from açaí seeds and curauá mucilage for methylene blue (MB) removal from aqueous solutions. Untreated (AWR, CWR) and phosphoric acid-treated (AAR, CAR) hydrochars were produced via hydrothermal carbonization and comprehensively characterized by Boehm titration, pH pzc , FTIR, SEM, TG/DTG, surface area, and elemental analyses. A 2 3 - factorial experimental design was employed to optimize adsorption conditions (pH, adsorbent dosage, and initial MB concentration). The statistical models explained more than 90% of the experimental variability, identifying pH as the most influential parameter and enabling MB removal efficiencies above 94% under alkaline conditions. Optimal adsorption was achieved at pH 10, with dosages of 0.05 g for açaí-derived hydrochars and 0.150 g for curauá-derived hydrochars at an initial MB concentration of 50 mg L −1 . Equilibrium data were well described by the Langmuir model, with maximum adsorption capacities of 22.65 (AWR), 27.95 (CWR), 27.10 (AAR), and 48.04 mg g −1 (CAR), demonstrating the positive effect of acid treatment on adsorption performance. Kinetic analysis followed the pseudo-second-order model, and thermodynamic results indicated a spontaneous and endothermic adsorption process. Reusability tests confirmed satisfactory stability over successive cycles. Overall, the results demonstrate that Amazonian agricultural residues can be effectively transformed into functional hydrochars for dye removal, combining sustainable waste valorization with statistically optimized adsorption performance.